1. Field of the Invention
The present invention relates to a method of thermally insulating a wellbore.
2. Description of the Prior Art
Thermal stimulation has become a recognized enhanced recovery technique in the production of petroleum crude oil from wells. One form of thermal stimulation in prevalent use in the industry is steam injection. Injection of high temperature steam into the production zone, however, is not without problems. In particular, when steam is injected through a tubing string, there is substantial transfer of heat to and through the well casing. This causes a substantial loss of thermal energy as the steam travels through the tubing string, thus resulting in less efficiency in the recovery process. In fact, thermal losses may be so great that steam will condense before reaching the oil-bearing formation, rendering the recovery process almost totally ineffective in reducing the viscosity of the reservoir oil. This problem is more severe in deeper wells, and means in many contexts of use that steam injection processes may be impractical in all except very shallow wells. Additionally, heat transfer to the well casing can induce stresses in the casing which causes fracture of the casing.
Heat loss through the walls of a wellbore is also experienced in the recovery of geothermal energy from deep within the earth. Such heat loss diminishes the usefulness of such resource and mitigates its use.
Heretofore, it has been proposed to prevent excessive heat loss by various means of insulating the wellbore.
Insulation has been proposed by gelling crude in the annulus between the tubing string and the casing, by forming a polymerized foam silicate around the tubing string, or by using manufactured insulated pipe joints. None of these approaches has proved satisfactory.
Additionally, it has been proposed to insulate a well with a vermiculite slurry, as taught in U.S. Pat. No. 3,650,327 to Burnside, and to insulate with a cement slurry which includes vermiculite or perlite, as taught in U.S. Pat. No. 3,360,046 to Johnson, et al. However, when subsequent operations necessitate removal of the tubing string, the cured cement slurry or vermiculite slurry, which cannot be completely dried of the carrier liquid, may hinder removal. And removal of the vermiculite slurry from the annular space may be difficult to accomplish because the slurry may be sufficiently viscous that it cannot be readily displaced.